1. Field of the Invention
This invention relates to a projection cathode ray tube in which an image is produced on a fluorescent layer and is projected on a screen on an enlarged scale through projecting lenses.
2. Description of the Related Art
U.S. Pat. No. 4,642,695 exemplifies a cathode ray tube which has been devised so as to improve low efficiency in gathering light beams into projection lenses from monochromatic cathode ray tubes in a projection type television set.
With an ordinary cathode ray tube, the light beams produced on the fluorescent layer have substantially complete divergence. On the contrary, with a projection type television set, only light beams that are produced on the fluorescent layer and have a divergent angle of 30.degree. or less are used, while the other light beams are not used. The above cited reference discloses that brightness of the image on the projection type television screen is much improved when more than 30% of the light beams produced from a certain luminescent point on the fluorescent layer is converged into the inside of a cone having the divergent angle of .+-.30.degree..
Japanese Patent Laid-Open Publication No. 273837/1986 exemplifies a projection cathode ray tube including a multi-layered interference filter, which is made of a high refractive index material and a low refractive index material superimposed one after another between an inner side of a face glass and a fluorescent layer. Tantalum pentaoxide (Ta.sub.2 O.sub.5) or titanium oxide (TiO.sub.2) is used as a high refractive index material, while silicon dioxide (SiO.sub.2) or magnesium fluoride (MgF.sub.2) is used as a low refractive index material. The interference filter contains more than five layers of the high and low refractive index materials superimposed one after another.
With the above cathode ray tubes, the optical distance nd of each layer (the letters n and d represent the refractive index and the thickness of the layer material) is 0.2 to 0.3 .lambda..sub.f, preferably 0.23 to 0.25 .lambda..sub.f in the cathode ray tube described in the above Japanese Reference (where .lambda..sub.f =p.times..lambda.: .lambda. stands for the central wavelength to be selected according to the luminous spectrum of the fluorescent material; p is a value between 1.18 and 1.32). Preferably the optical distance should be approximately .lambda..sub.f /4.
It is usually necessary to wash the inner surface of the face glass prior to applying a fluorescent material to the face glass. Hydrofluoric acid (HF) having a density of several per cent is used for this purpose.
Further, if the face glass suffers from an inconvenience such as peeling of the fluorescent material, film or evaporated aluminum, or roughened surface, the fluorescent layer is removed by a process referred to as a salvaging process, thereby recycling the glass material.
However one layer of the interference filter of the conventional projection cathode ray tube is very thin, i.e., about 1000 to 2000 .ANG. thick. When the face glass were washed with hydrofluoric acid, interference filter layers would peel off. Therefore such face glass is washed only by water or detergent. The face glass which is not washed with hydrofluoric acid has been problematic in that it suffers from peeling fluorescent materials or dusts. It is also disadvantageous that the glass material cannot be reused since it cannot be subject to the salvaging process.